Anti-Shock Optical Recording and Reproducing Device

ABSTRACT

The present invention relates to an optical recording and reproducing device comprising an optical pick-up unit, position means for moving roughly the optical pick-up unit with respect to a desired position on an optical disc, an actuator for moving the optical pick-up unit relative to the position means in a radial direction of the optical disc, and a servo processor for delivering a control signal (u(k)) from a measured radial error signal (x(k)) derived from the optical pick-up unit. Said servo processor further comprises: —a state estimator (SEST) for delivering an estimated radial error signal ( ) and a predicted radial error signal ( ) on the basis of the measured radial error signal (x(k)) and of the control signal (u(k)); and—a shock detector (SDET) for delivering a shock indication (S_out) on the basis of the estimated radial error signal, of the predicted radial error signal, and of a sum (CA(k)) of the signals delivered by the at least two regions of the optical sensor.

FIELD OF THE INVENTION

The present invention relates to an optical recording and reproducingdevice.

The present invention further relates to a servo processor for use in anoptical recording and reproducing device and a processing method for usein an optical recording and reproducing device.

The present invention may be used, for example, in optical data drives,portable and mobile applications and recordable drives (CDRW, DVDRW)where the anti-shock performance is of great importance.

BACKGROUND OF THE INVENTION

There are conventionally two ways of improving the optical drive'santi-shock performance, the first one through mechanism suspensiondesign and the second one through shock robustness improvement of servosystem.

Conventional suspension design can efficiently absorb influence of shockin the low frequency range, usually below 100 Hz. But the disturbanceinfluence to the tracking performance at a comparatively higherfrequency range has also to be suppressed through extra servo controllerdevice, conventionally by increasing the servo loop bandwidth. However,increasing servo loop bandwidth increases the loop sensitivity to noisesuch as disc defects in the low frequency range. That is why detectingaccurately and promptly shock becomes critical for applications based onanti-shock measurements. U.S. Pat. No. 6,163,429 discloses ashock-detection method used in an optical disc drive. Saidshock-detection method is based on the use of the radial error signal ofthe laser beam. This radial error signal is filtered in a correlationfilter and fed into a comparator, which compares the error signal with apredetermined value and generates a binary output indicating whether ashock has or has not been detected.

However, this method is not able to detect properly a sudden rise in theradial error signal caused by disturbances such as disc defects.Moreover, it may not be fast enough to let the servo controller react.

SUMMARY OF THE INVENTION

It is an object of the invention to propose an optical recording andreproducing device, which is less sensitive to shock during playing andrecording than the one of the prior art.

To this end, the optical recording and reproducing device in accordancewith the invention comprises:

an optical pick-up unit including an optical sensor divided into atleast two regions;

a servo processor for delivering a control signal from a measured radialerror signal delivered by the optical pick-up unit;

said servo processor further comprising:

a state estimator for delivering an estimated radial error signal and apredicted radial error signal on the basis of the measured radial errorsignal and of the control signal; and

a shock detector for delivering a shock indication on the basis of theestimated radial error signal, of the predicted radial error signal, andof a sum of the signals delivered by the at least two regions of theoptical sensor.

The present invention is based on the fact that the sum signal derivedfrom the optical sensor remains at the same level during a shock and inthe absence of shock. But during disc defects, said sum signal eitherdrops or increases due to a decrease or increase, respectively, of lightintensity reflected by a disc and caused by the disc defects. As aconsequence, occurrence of a shock can be accurately and promptlydetected thanks to the estimated radial error signal, the predictedradial error signal, and said sun signal.

The present invention also relates to the servo processor for use insuch an optical recording and reproducing device. It finally relates toa processing method for use in said optical recording and reproducingdevice.

These and other aspects of the invention will be apparent from and willbe elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail, by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a block diagram of an architecture of a servo processor inaccordance with the invention;

FIG. 2 is a block diagram of a state estimator;

FIG. 3 is a block diagram of a shock detector in accordance with theinvention;

FIG. 4 is a block diagram of a memory loop; and

FIG. 5 is a curve representing a frequency response spectrum of a radialerror signal with shock and without shock.

DETAILED DESCRIPTION OF THE INVENTION

The present invention proposes an optical recording and reproducingdevice (e.g. an optical drive), which is more robust to shock.

In an embodiment of the optical recording and reproducing deviceaccording to the invention, the device comprises an optical pick-up unitmovably mounted on positioning means for roughly positioning the opticalpick-up unit with respect to a desired position on a disc. Thepositioning means is usually implemented by a sledge. The opticalpick-up unit includes a semiconductor laser for delivering a laser beam,an objective lens focusing the laser beam on the disc and an opticalsensor (e.g. laser diodes) detecting the light reflected from said disc.The optical sensor is divided into at least two regions, a front halfregion and a rear half region. As an example, the optical sensor isdivided into four regions, two regions being arranged to receive lightreflected from the front half of a laser spot formed on the disc whilethe two other regions are arranged to receive light reflected from therear half of the laser spot. An adder is then adapted to generate a sumsignal from a sum of each signal generated by one of the four regions inresponse to the reflected light beam.

During operation of the device, the optical pick-up unit is controlledby an actuator for moving the optical pick-up unit relative to thepositioning means in a radial direction of the disc so as to fineposition the optical pick-up unit with respect to the desired positionon the disc. The actuator comprises a servo-tracking controller foranti-shock control of the recording and reproducing device, asillustrated in FIG. 1. Said servo-tracking controller SC is implementedwithin a servo digital servo processor DSP. The digital servo processorcomprises a state estimator SEST for estimating an estimated radialerror x(k) and an estimated velocity v(k) of the actuator. Said stateestimator has an input responsive to a measured radial error x(k)derived from the optical pick-up unit and another input responsive to acontrol signal u(k). The measure radial error x(k) is generated by a3-beam method know to a person skilled in the art and depicted forexample in the book entitled “The CD-ROM Drive—A Brief SystemDescription”, by Sorin G. Stan, Kluwer Academic Publishers, 1998.

Said digital servo processor also comprises a shock detector SDET forsupplying a shock interrupt signal under control of information suppliedby the state estimator to an input of the shock detector. Said shockdetector has multiple inputs and a single output. The digital servoprocessor finally comprises the servo controller SC for supplying thecontrol signal u(k) to the actuator based on information derived fromthe shock detector which receives information from the state estimator,the servo controller also feeding back said control signal to the stateestimator. The state estimator and the shock detector run at the servoprocessor clock frequency of 22 kHz.

The state estimator is adapted to estimate an entire state based on ameasurement of one of the state elements. For the digital servoprocessor, the state estimator estimates the actuator position, velocityand the control signal based on the measurement of the radial errorsignal. FIG. 2 represents a conventional state estimator. The stateestimator estimates a radial laser beam position error signal at a timek, x(k)and a corresponding estimated velocity v(k). The estimated statesare then delivered to the shock detector to predict the occurrence ofshock.

The state estimator mainly comprises two blocks: a state observer OBSand a state predictor PRED. A measured radial error signal x(k) at atime k is provided to the state observer block which estimates a currentstate of the actuator including the estimated radial error signal x(k)and the estimated velocity v(k) of the actuator, as follows:

x (k)={circumflex over (x)}(k+1)/z+L _(res)(x(k)−{circumflex over(x)}(k+1)/z)

v (k)={circumflex over (v)}(k+1)/z+L _(v)(v(k)−{circumflex over(v)}(k+1)/z)

where L_(res) and L_(v) are the estimator gain provided by a linearquadratic regulator LQR method according to a principle known to aperson skilled in the art. The estimated states together with thecurrent control signal u(k) are delivered to the state predictor topredict the states {circumflex over (x)}(k+1) and v(k+1) of the actuatorat a next time k+1, as follows:

{circumflex over (x)}(k+1)=a11. x (k)+a12. v (k)+b1.u(k)

{circumflex over (v)}(k+1)=a21. x (k)+a22. v (k)+b1.u(k)

where a11, a12, a21, a22 and b1 are constant calculated from thecharacteristics of the actuator. The use of state estimator allows toquickly detecting the occurrence of a shock.

The shock detector mainly comprises 4 parts: a set of 2 band-passfilters, a set of 2 memory loops, and a set of 3 comparators. FIG. 3shows the implementation of such a shock detector.

The band-pass filters IIR1 and IIR2 are preferably of the infiniteimpulse response IIR type. Said filters are, for example, a 4th orderElliptic filter defined by the following function H:

${H(z)} = \frac{b_{1} + {b_{2}z^{- 1}} + {b_{3}z^{- 2}} + {b_{4}z^{- 3}} + {b_{5}z^{- 4}}}{a_{1} + {a_{2}z^{- 1}} + {a_{3}z^{- 2}} + {a_{4}z^{- 3}} + {a_{5}z^{- 4}}}$

where a1 to a5, b1 to b5 are design coefficients that can be adjustedaccording to the dynamic characteristics of the optical drive.

Each band-pass filter is adapted to receive the estimated radial errorsignal x(k) or predicted radial error signal {circumflex over (x)}(k+1)and to deliver a corresponding filtered radial error signal FIL. It isdesigned such that it filters the influence of shock in the lowfrequency range, for example lower than 100 Hz, and that it filters highfrequency noise, for example higher than 1000 Hz. In the case of aPhilips Mercury 2 DVD drive, the shock influence on the radial errorsignal in the low frequency range can be absorbed by the suspensiondesign. It is the shock energy in a higher frequency range that excitedthe actuator and caused a sudden rise in the position error. FIG. 5shows the frequency response spectrum of a radial error signal whendifferent levels of shock occurred. It can be seen that the maininfluence of shock that lead to a radial laser beam off track lies inthe middle frequency range, more exactly between 20 Hz and 1000 Hz. Thefilter coefficients are for example set as follows:

a1=1.0, a2=−2.0981, a3=1.6417, a4=−0.8443, a5=0.3130

b1=0.2796, b2=−0.1471, b3=−0.2637, b4=−0.1471, b5=0.2796

so that the radial error information in the frequency range lyingbetween 100 Hz and 1000 Hz is extracted.

The memory loops LOOP1 and LOOP2 are connected in series with theband-pass filters IIR1 and IIIR2, respectively. Each memory loop isadapted to receive the filtered radial error signal FIL and to deliver acumulative radial error signal SUM. It is, for example, a 3-tap FIFO.Such a memory loop is able to memorize and to add 4 consecutive filteredradial error signals for a good and reliable detection of the occurrenceof sudden changes in the radial error signal. FIG. 4 shows animplementation of such a memory loop, where the block D represents adelay line. It will be apparent to a person skilled in the art that thememory loop can use more or less than 4 consecutive radial errorsignals. The choice of the memory loop order (i.e. the number ofconsecutive filtered error signals to be added) depends on the signalprocessing frequency used compared with the shock.

The memory loop is also designed in order to prevent shock-out flagoscillation when the position error signal vibrates too much during theshock (i.e. the shock detection flag is frequently on and off when theradial error signal crosses zero during one shock). The use of a memoryloop allows to keeping the shock indicator signal stable during shocks,instead of an agitated high-low-high-low shock on flag caused by highfrequency oscillation excited by the shocks.

First and second comparators COMP1 and COMP2 connected in series withthe memory loops LOOP1 and LOOP2 compare the cumulative radial errorsignal SUM with predetermined values as shown in the 2 followingconditions (1) and (2):

SUM≧V_(high limit1)and SUM not≦V_(low limit)   (1)

or

SUM≧V_(high limit2)and SUM not≦V_(low limit)   (2)

where V_(high limit1), V_(high limit2) and V_(low limit) are designparameters depending on the settings of the band-pass filters and of theactuator signal sensitivity.

The outputs from the two comparators COMP1 and COMP2 are delivered to alogic OR operator. Whenever one of the 2 following condition is met, abinary output 1 is generated at the output of the OR operator toindicate a sudden increase of the position error signal due todisturbance.

The design parameters are determined in such a way that the parameterV_(high limit1) is equivalent to 20% of the track pitch value and theparameter V_(high limit2) is about 25% of the track pitch value. Thesevalues have been determined experimentally in the particular case of acumulative radial error signal corresponding to the sum of 4 consecutivefiltered radial error signals. The parameter V_(low limit) is the lowlimit of the comparator. Said parameter is used together with the memoryloop to prevent wrong triggering of the binary output 0 when the radialerror signal passes through zero and at the same time indicates that thelaser spot is back to a precise tracking. The parameter V_(low limit) isequivalent to 1% of the track pitch value. These values can be adjustedand finalized through testing of a limited number of optical drives ofthe same type.

A third comparator COMP3 then calculates the variation of the sum signalCA(k) of the optical sensor. Conventionally, if the following condition:

|CA(K)−CA(k−1)|/|CA(k−1)|>5%   (3)

is met, it means there is a light intensity deviation caused by discdefects. In this case, the third comparator will then output a binaryvalue equal to 0. Otherwise, a binary output of 1 is generated.

The outputs from the logic OR operator and of the third comparator arethen provided to a logic AND operator delivering an output signal S_out.If the output of the AND operator is a binary 1, i.e. condition (1) or(2) is fulfilled, and if condition (3) is fulfilled, i.e. a defect isdetected by the third comparator, then the shock detector finallyoutputs binary 1 indicating a detection of shock. As a consequence, thesum signal is used to distinguish a sudden rise of radial error signalis caused by disc defects or shock.

Any reference sign in the following claims should not be construed aslimiting the claim. It will be obvious that the use of the verb “tocomprise” and its conjugations do not exclude the presence of any othersteps or elements besides those defined in any claim. The word “a” or“an” preceding an element or step does not exclude the presence of aplurality of such elements or steps.

1. An optical recording and reproducing device comprising: an opticalpick-up unit including an optical sensor divided into at least tworegions; a servo processor for delivering a control signal (u(k)) from ameasured radial error signal (x(k)) delivered by the optical pick-upunit; said servo processor further comprising: a state estimator (SEST)for delivering an estimated radial error signal ( x(k)) and a predictedradial error signal ({circumflex over (x)}(k+1)) on the basis of themeasured radial error signal (x(k)) and of the control signal (u(k));and a shock detector (SDET) for delivering a shock indication (S_out) onthe basis of the estimated radial error signal, of the predicted radialerror signal, and of a sum (CA(k)) of the signals delivered by the atleast two regions of the optical sensor.
 2. A servo processor for use inan optical recording and reproducing device, said servo processor beingadapted to deliver a control signal (u(k)) from a measured radial errorsignal (x(k)) delivered by an optical pick-up unit, said servo processorcomprising: a state estimator (SEST) for delivering an estimated radialerror signal ( x(k)) and a predicted radial error signal ({circumflexover (x)}(k+1)) on the basis of the measured radial error signal (x(k))and of the control signal (u(k)); and a shock detector (SDET) fordelivering a shock indication (S_out) on the basis of the estimatedradial error signal, of the predicted radial error signal, and of a sum(CA(k)) of the signals delivered by the at least two regions of theoptical sensor.
 3. A servo processor as claimed in claim 2, wherein theshock detector comprises two band-pass filters (IIR) for filtering theestimated radial error signal ( x(k)) and the predicted radial errorsignal ({circumflex over (x)}(k+1)) and designed such that they filterinfluence of shock in the low frequency range and that they filter highfrequency noise.
 4. A servo processor as claimed in claim 3, wherein theband-pass filters are of the infinite impulse response type.
 5. A servoprocessor as claimed in claim 3, wherein the shock detector furthercomprises two memory loops (LOOP1,LOOP2), each memory loop beingconnected in series with one of the band-pass filters and being adaptedto deliver a cumulative radial error signal (SUM) from a set ofconsecutive filtered radial error signal (FIL) delivered by thecorresponding band-pass filter. 6 A servo processor as claimed in claim5, wherein the shock detector further comprises two comparators(COMP1,COMP2), each comparator being connected in series with one of thememory loops and being adapted to compare the cumulative radial errorsignal with predetermined thresholds so as to indicate a sudden increaseof the radial error signal due to a disturbance.
 7. A servo processor asclaimed in claim 6, wherein the shock detector further comprises anadditional comparator COMP3 for calculating variation of the sum signal(CA(k)) so as to indicate a light intensity deviation caused by a discdefect, a shock being detected if the additional comparator indicates adisc defect and if one of the two comparators indicates a suddenincrease of the radial error signal.
 8. A processing method for use inoptical recording and reproducing device, said method comprising thesteps of: state estimating, adapted to deliver an estimated radial errorsignal ( x(k)) and a predicted radial error signal ({circumflex over(x)}(k+1)) on the basis of the measured radial error signal (x(k)) andof the control signal (u(k)); and shock detecting, adapted to deliver ashock indication (S_out) on the basis of the estimated radial errorsignal, of the predicted radial error signal, and of a sum (CA(k)) ofthe signals delivered by the at least two regions of the optical sensor.